U.S. Pat. No. 4,625,993 discloses an active suspension system for an automotive vehicle. The active suspension system operates between the sprung (wheels) and unsprung (chassis) masses of the vehicle. It achieves and maintains (with no vehicle operator inputs or responses) a desired attitude, i.e., minimizes the motion, acceleration and vibration of the passenger compartment caused by road and vehicle dynamics, to improve vehicle handling, reduce noise, improve ride, and improve stability. These features of vehicle dynamics are achieved by controlling hydraulic suspension units located at the vehicle wheels. The system comprises a hydraulic circuit with hydraulic actuators and valves, a plurality of sensors, and an electrical control system. The hydraulic circuit communicates a source of pressurized hydraulic fluid with hydraulic actuators at the suspension units. The sensors sense acceleration and impact inputs to the vehicle, and the electrical control system controls the flow of hydraulic fluid in the actuators in response to the sensed inputs.
The suspension units in the active suspension system disclosed in the '993 patent each comprise a hydraulic actuator and a spring acting in parallel between a vehicle wheel and the vehicle chassis. An electro-hydraulic servo control valve associated with an actuator controls the flow of hydraulic fluid which effects movement of a piston in such actuator. Sensors at each suspension unit sense displacement between the wheel and the chassis, the load on the actuator, and acceleration of the wheel hub. Other sensors carried on the vehicle sense speed and acceleration of the vehicle.
The electrical control system responds to outputs from the sensors to provide displacement demand signals for each suspension unit so as to control static and dynamic displacement of the sprung and unsprung masses. The electrical control system also provides signals representing actual displacement for each suspension unit. The control system compares the displacement demand signals with the actual displacement signals to produce error signals. Each servo valve in the hydraulic circuit is operated in accordance with the associated error signal to cause the actual displacement of the piston in the associated actuator to equal the desired displacement. The active suspension system thus controls the positions of the pistons to achieve a desired vehicle attitude by operating the servo valves.
Servo valves in known active suspension systems are durable and have proved themselves to be reliable. One particular known servo valve for an active suspension system uses a magnetic coil to control displacement of a reed. The amount of displacement of the reed is functionally related to the amount of current applied to the coil. The reed is operatively located between two pilot control orifices. The pilot control orifices are in fluid communication with associated sides of a spool valve. The position of the spool valve, which is a function of the opposed pilot pressures, controls the fluid flow to an associated actuator. The position of the reed controls the pilot pressures. Such a servo valve is referred to as voltage controlled servo valve. While such voltage controlled servo valves have been satisfactory in an active suspension system, they are expensive.